The utilization of adhesives and the bonding of wood has its orgin in the early history of man. The first adhesives for bonding wooden articles were those which occurred in nature, for example, glue from animal protein by-products, vegetable starch, caseins, soybeans, blood, blood-albumin and the like. As effective as these adhesives may be--indeed some are still used today--such naturally occurring adhesives do not always provide strengths or properties that will meet present day specifications. This is especially true in the wood industry where bond strength and resistance to moisture is a primary concern. The wood industry, particularly the plywood industry, has turned to the newer synthetic adhesives which are chemically reactive, such as amino or phenolic resins, for example, urea phenol, melamine, resorcinol and the like. Such thermosetting resin adhesives develop bond strength through a chemical reaction in the actual bonding process. Generally speaking, such chemically reactive thermosetting resin adhesives employ a monomer or low molecular weight polymer admixed with a catalyst or an initiator, and which additionally may contain other materials, prior to being spread in liquid form on the adherend surface to be bonded. The joint is formed by applying the other adherend surface, usually under pressure, and allowing the adhesive to "cure". The "curing" is the chemical reaction which progresses whether at room temperature or at some elevated temperature for a period of time sufficient for the chemical reaction to go essentially to completion. The synthetic thermosetting resins which are chemically reactive adhesives suffer from the disadvantages of greater cost than naturally occurring adhesives, limited shelf and liquid pot lives when prepared for use, the length of life depending on the ratio of catalyst to resin and temperatures at which the adhesives are stored or used. Another disadvantage which is especially true in the plywood industry is that the chemically reactive synthetic resin adhesives have relatively long "curing" times. This disadvantage is especially troublesome because the press-curing in the manufacture of plywood is recognized as a natural bottleneck in a well designed plant for continuous plywood manufacture. Curing temperatures from 80.degree.-140.degree. C. are used, depending on the type and application of the plywood, for periods of 5-10 minutes under pressures chosen to bring about compaction, i.e., all plies being brought into intimate contact, and, above that, 4 percent compression.
Our recently issued U.S. Pat. No. 3,855,054 teaches an article and process for bonding two wood adherend surfaces together with an adhesive having a high bond strength, high resistance to moisture and relatively short "curing" time. The adhesive described is solidified sulfur which may additionally contain conventional fillers such as wood flour, sawdust, fine sand, fly ash, inert clays, metallic filings, wools or powders, pigments, such as iron oxide, calcium chromate and barium chromate and up to 25 weight percent plasticizers, such as polyethylene tetrasulfide, polydiacetic tetrasulfide, dimercaptobutane, diallyl tetrasulfide, mercaptoethylene and cyclohexane thiols. Wood bonded with the sulfur adhesives of our U.S. Pat. No. 3,855,054 patent has, in addition to short curing time, resistance to moisture. However, conventional applicator equipment cannot be used as it is applied in powder or stiff paste form.